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Code 21 - Coolant Temperature Sensor

NOTE: Refer to illustration for ECU connector and terminal identification. See Figure .
  1. Turn ignition off. Disconnect coolant temperature sensor connector. Measure resistance between coolant temperature sensor terminals. See the COOLANT TEMPERATURE SENSOR RESISTANCE  table. If resistances are not to specification, replace coolant temperature sensor. If resistances are to specification, go to next step.
    COOLANT TEMPERATURE SENSOR RESISTANCE

    Temperature °F (°C) Ohms
    68°F (20°C) 2000-3000
    176°F (80°C) 300-400
  2. Turn ignition on. Measure voltage between ECU connector B48, terminal No. 7 (backprobe) and ground. If .6-4.5 volts are present, check ECU terminals for poor contact. If .6-4.5 volts are not present, go to next step.
  3. Ensure ignition is off. Disconnect ECU and coolant temperature sensor connectors. Measure resistance between ECU connector B48, terminal No. 7 and coolant temperature sensor terminal No. 1, then measure resistance between ECU connector B48, terminal No. 21 and coolant temperature sensor terminal No. 2. Zero ohms should be present in both cases. Repair harness if not zero ohms.
  4. Measure resistance between coolant temperature sensor harness terminals and ground. At least one megohm should be present. If resistance is to specification, repair ECU terminals for poor contact or replace ECU. If resistance is not to specification, repair harness and/or connector.
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